(1) Field of the Invention
THIS INVENTION relates to gas separation systems utilising molecular sieves and is especially but not exclusively connected with the employment of such systems in the aviation field for obtaining oxygen-enriched air as a breathable gas for aircrew.
(2) Description of the Prior Art
In a gas separation system utilising molecular sieves, a feed gas mixture is caused to flow through a bed of molecular sieve material having the capability of retaining unwanted constituent(s) of the gas mixture while passing the wanted constituent(s): for instance, in the separation of oxygen from air a molecular sieve material type 5A as is available, for example, from BEYER A.G. or UNION CARBIDE CORPORATION may be used to hold back nitrogen and to pass oxygen to provide a delivery of a product gas that is approximately 95% oxygen. In the operation of such a system, the sieve material becomes progressively saturated with the retained gas constituent(s), e.g. nitrogen, and eventually becomes incapable of retaining further amounts of such constituent(s). The sieve material of the bed, therefore, has to be regenerated from time to time to purge it of the retained constituent(s). Typically regeneration is effected by flushing the bed, usually in the reverse direction, with the product gas while the outflow from the bed is discharged, e.g., to atmosphere.
To provide for continuous delivery of the desired product gas, the system may include a reservoir for this and from which product gas is drawn for flushing the bed for regeneration purposes. Additionally or alternatively, the system may include a plurality of beds arranged to be utilised and regenerated in an overlappying sequence to provide for continuous delivery of a required product gas.
A common manner of operating a gas separation system employing one or more molecular sieve beds involves charging the or each bed with feed gas mixture--e.g. air--under pressure and continuing the feed to the bed to maintain the charge pressure during delivery of the required product gas constituent(s) to use or storage. When the bed approaches saturation, the feed is discontinued and the bed vented to release the charge pressure, whereafter the bed is purged. For convenience, a system operated in this manner is referred to herein as a "pressure sieving gas separation system."
In aircraft applications it is normally required that the oxygen concentration (partial pressure) in the breathable gas supplied to aircrew shall be related to cabin altitude, i.e. to the ambient pressure obtaining within their enclosure.
Hitherto in proposed aircraft applications of molecular sieve type gas separation systems, such systems have been treated as a source of substantially pure oxygen to be utilised in the manner traditional in aircrew breathable gas suppy systems: that is to say, the substantially pure oxygen product gas delivered by the separation system is proposed to be diluted with air to provide a breathable gas mixture of the required oxygen partial pressure, in a manner analogous to the utilisation of the oxygen obtained from sources such as high pressure storage bottles and liquid oxygen converters.
The utilisation of a product gas that is substantially pure oxygen for producing oxygen-enriched air as breathable gas involves the use of appropriate mixing valves and related control devices to derive the desired breathable gas mixture. Such valves and control devices tend to be heavy and to be potentially unreliable through their inclusion of capsules and the like for sensing ambient pressure changes.